diff --git a/std/StringTools.hx b/std/StringTools.hx
index d8efdd8b653..ea6f3923d69 100644
--- a/std/StringTools.hx
+++ b/std/StringTools.hx
@@ -464,7 +464,7 @@ class StringTools {
 		#elseif java
 		return (index < s.length) ? cast(_charAt(s, index), Int) : -1;
 		#elseif js
-		return (cast s).charCodeAt(index);
+		return (cast s : js.lib.NativeString).charCodeAt(index);
 		#elseif python
 		return if (index >= s.length) -1 else python.internal.UBuiltins.ord(python.Syntax.arrayAccess(s, index));
 		#elseif hl
@@ -501,7 +501,7 @@ class StringTools {
 		#elseif java
 		return cast(_charAt(s, index), Int);
 		#elseif js
-		return (cast s).charCodeAt(index);
+		return (cast s : js.lib.NativeString).charCodeAt(index);
 		#elseif python
 		return python.internal.UBuiltins.ord(python.Syntax.arrayAccess(s, index));
 		#elseif hl
diff --git a/std/js/_std/EReg.hx b/std/js/_std/EReg.hx
index ae91f2ff697..c2323bbd212 100644
--- a/std/js/_std/EReg.hx
+++ b/std/js/_std/EReg.hx
@@ -19,6 +19,8 @@
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  */
+import js.lib.NativeString;
+
 @:coreApi class EReg {
 	var r:HaxeRegExp;
 
@@ -84,7 +86,7 @@
 	}
 
 	public inline function replace(s:String, by:String):String {
-		return (cast s).replace(r, by);
+		return (cast s : NativeString).replace(r, by);
 	}
 
 	public function map(s:String, f:EReg->String):String {
@@ -112,7 +114,7 @@
 	}
 
 	public static inline function escape(s:String):String {
-		return (cast s).replace(escapeRe, "\\$&");
+		return (cast s : NativeString).replace(escapeRe, "\\$&");
 	}
 
 	static var escapeRe = new js.lib.RegExp("[.*+?^${}()|[\\]\\\\]", "g");
diff --git a/std/js/_std/HxOverrides.hx b/std/js/_std/HxOverrides.hx
index 8f55624dec0..e234c34864b 100644
--- a/std/js/_std/HxOverrides.hx
+++ b/std/js/_std/HxOverrides.hx
@@ -56,7 +56,7 @@ class HxOverrides {
 
 	@:pure
 	static function cca(s:String, index:Int):Null<Int> {
-		var x = (cast s).charCodeAt(index);
+		var x = (cast s : js.lib.NativeString).charCodeAt(index);
 		if (x != x) // fast isNaN
 			return js.Lib.undefined; // isNaN will still return true
 		return x;
diff --git a/std/js/lib/NativeString.hx b/std/js/lib/NativeString.hx
new file mode 100644
index 00000000000..70664aa5f67
--- /dev/null
+++ b/std/js/lib/NativeString.hx
@@ -0,0 +1,34 @@
+package js.lib;
+
+/**
+	Native javascript string.
+**/
+@:native("String")
+extern class NativeString {
+	/**
+		The `charCodeAt()` method of String values returns an integer between 0
+		and 65535 representing the UTF-16 code unit at the given index.
+
+		If `index` is out of range of `0` – `str.length - 1`, returns `NaN`.
+
+		See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charCodeAt
+	**/
+	@:pure function charCodeAt(index:Int):Int;
+
+	/**
+		The `replace()` method of String values returns a new string with one,
+		some, or all matches of a pattern replaced by a replacement.
+
+		The pattern can be a `String` or a `js.lib.RegExp`, and the replacement
+		can be a string or a function called for each match.
+
+		If pattern is a string, only the first occurrence will be replaced. The
+		original string is left unchanged.
+
+		See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace
+	**/
+	@:overload(function(pattern:RegExp, replacement:String):String {})
+	@:overload(function(pattern:RegExp, replacement:String->String):String {})
+	@:overload(function(pattern:String, replacement:String->String):String {})
+	@:pure function replace(pattern:String, replacement:String):String;
+}
diff --git a/tests/server/src/cases/issues/Issue11904.hx b/tests/server/src/cases/issues/Issue11904.hx
new file mode 100644
index 00000000000..388d7e7173c
--- /dev/null
+++ b/tests/server/src/cases/issues/Issue11904.hx
@@ -0,0 +1,30 @@
+package cases.issues;
+
+import haxe.display.Diagnostic;
+
+class Issue11904 extends TestCase {
+	function test(_) {
+		vfs.putContent("Issue11904.hx", getTemplate("issues/Issue11904.hx"));
+		var args = ["-main", "Issue11904", "--js", "no.js", "--no-output"];
+		runHaxe(args);
+		runHaxeJson([], ServerMethods.Invalidate, {file: new FsPath("Issue11904.hx")});
+		runHaxeJsonCb(args, DisplayMethods.Diagnostics, {file: new FsPath("Issue11904.hx")}, res -> {
+			Assert.equals(1, res.length);
+			Assert.equals(2, res[0].diagnostics.length);
+
+			function check<T>(d:Diagnostic<T>) {
+				switch (d.kind) {
+					case ReplaceableCode:
+						Assert.equals("Unused variable", d.args.description);
+
+					case _:
+						// trace(d);
+						Assert.fail("Unexpected diagnostics kind: " + d.kind);
+				}
+			}
+
+			var diag = res[0].diagnostics;
+			for (d in diag) check(d);
+		});
+	}
+}
diff --git a/tests/server/test/templates/issues/Issue11904.hx b/tests/server/test/templates/issues/Issue11904.hx
new file mode 100644
index 00000000000..0a3fc17219c
--- /dev/null
+++ b/tests/server/test/templates/issues/Issue11904.hx
@@ -0,0 +1,9 @@
+using StringTools;
+
+@:nullSafety
+class Issue11904 {
+	static function main() {}
+	static function extractReturnType(hint:String):Void {
+		for (i => code in hint) {}
+	}
+}